The present invention generally relates to a communication cable for use in a plenum and, in particular, relates to one such communication cable having a first plurality of twisted pairs of electrical conductors having a first insulating material about each electrical conductor thereof and a second plurality of twisted pairs of electrical conductors having a second insulating material about each electrical conductor thereof.
As communications and communication services have increased, it has become necessary to provide communication cables in larger and larger numbers. This is particularly true in office buildings where more and more communication services are being demanded. Typically, rather than rewire an existing building, it has been found more economical to provide the needed communication services by running the communication cables in plenums. In general, a plenum is defined as a compartment or chamber to which one or more air ducts are connected and which forms part of the air distribution system. Generally, in existing buildings, plenums are readily formed by providing drop ceilings, which is typically a return air plenum, in a facility being rewired. Another alternative is to create a plenum beneath a raised floor of a facility.
From the above it is readily understood why it would be very advantageous to utilized a wiring scheme within these fairly accessible places. However, since these plenums handle environmental air, considerable concern regarding a fire incidence is addressed in the National Electrical Code by requiring that communication cables for use in plenums pass a stringent flame and smoke evaluation. Consequently, in the manufacture of communication cables the fire resistance ratings which allow for installation within certain areas of a building are of primary importance.
Currently, communication cables for use in plenums must meet the requirements of the Underwriter's Laboratory UL Standard 910 which is a Test Method For Fire and Smoke Characteristics of Cables Used In Air-Handling Spaces. This is a well known test performed in a modified Steiner Tunnel (Steiner Tunnel). During the test, a single layer of 24 foot lengths of cable are supported on a one foot wide cable rack which is filled with cables. The cables are ignited with a 300,000 Btu/hr methane flame located at one end of the furnace for a duration of 20 minutes. Flame spread is aided by a 240 ft/minute draft. Flame spread is then monitored through observation windows along the side of the tunnel while concurrently monitoring smoke emissions through photocells installed within the exhaust duct. This is a severe test that to date has been passed by communication cables using premium materials such as low smoke materials, for example, Fluroethylenepropylene (FEP), Ethylene-chlorotrifluoroethylene (ECTFE), or Polyvinylidene fluoride (PVDF). In general, cables meeting this test are approximately three times more expensive than a lower rated cable designed for the same application. However, communication cables failing this test must be installed within conduit, thereby eliminating the benefits of an economical, easily relocatable cable scheme.
In general, the manufacture of communication cables are well known, for example, U.S. Pat. No. 4,423,589, issued to Hardin et al. on Jan. 3, 1984 discloses a method of manufacturing a communications cable by forming a S plurality of wire units by advancing groups of twisted wire pairs through twisting stations. Further, U.S. Pat. No. 4,446,689 issued to Hardin et al. on May 8, 1984 relates to an apparatus for manufacturing a communications cable wherein disc frames are provided with aligned apertures in which faceplates movably mounted. During operation, the faceplates are modulated in both frequency and amplitude.
The current materials for use in communications are also well known, for example, U.S. Pat. No. 5,001,304 issued to Hardin et al. on Mar. 19, 1991 relates to a building riser cable having a core which includes twisted pairs of metal conductors. Therein the insulating covers are formed from a group of materials including polyolefin. It should be noted however, that all of the insulating covers are the same and that the flame test used for riser cables is much less severe than the flame test used for plenum cables.
U.S. Pat. No. 5,024,506 issued to Hardin et al. on Jun. 18, 1991 discloses a plenum cable that includes non-halogenated plastic materials. The insulating material about the metallic conductors is a polyetherimide. Again the insulating material is the same for all of the conductors. Further, in U.S. Pat. No. 5,074,640 issued to Hardin et al. on Dec. 24, 1991 a plenum cable is described that includes an insulator containing a polyetherimide and an additive system including an antioxidant/thermal stabilizer and a metal deactuator. As is the convention, the insulator is the same for all of the metallic conductors.
U.S. Pat. No. 5,202,946 issued to Hardin et al. on Apr. 13, 1993 describes a plenum cable wherein the insulation includes a plastic material. The insulation is the same for all of the conductors within the plenum cable. European Patent 0 380 245 issued to Hardin et al. describes another plenum cable having insulation about the metallic conductors that, in this case, is a plastic material including a polyetherimide. As is the convention the insulation is the same for all of the conductor.
Further, U.S. Pat. No. 4,941,729 describes a cable that is intended as a low hazard cable. This patent describes a cable that includes a non-halogenated plastic material. Similarly, U.S. Pat. No. 4,969,706 describes a cable that includes both halogenated and non-halogenated plastic materials. In both patents the insulating material about the twisted pairs of conductors is the same for each cable.
U.S. Pat. No. 4,412,094 issued to Dougherty et al. on Oct. 25, 1983 relates to a riser cable having a composite insulator having an inner layer of expanded polyethylene and an outer layer of a plasticized polyvinyl chloride. All of the conductors include the same composite insulator.
U.S. Pat. No. 4,500,748 issued to Klein on Feb. 19, 1985 relates to a flame retardant plenum cable wherein the insulation and the jacket are made from the same or different polymers to provide a reduced amount of halogens. This reference tries to predict, mathematically, the flame spread performance of cables within the Steiner tunnel. The method does not consider configurations of designs. Further, synergistic effects or the smoke generation of the design against UL 910 test requirements are not addressed. In each embodiment, the insulation is the same for all of the conductors.
U.S. Pat. No. 4,605,818 issued to Arroyo et al. on Aug. 12, 1986 relates to a flame retardant plenum cable wherein the conductor insulation is a polyvinyl chloride plastic provided with a flame retardant, smoke suppressive sheath system. As is common throughout the known communication cables the conductor insulation is the same for all of the conductors.
U.S. Pat. No. 4,678,294 issued to Angeles on Aug. 18, 1987 relates to a fiber optic plenum cable. The optical fibers are provided with a buffer layer surrounded by a jacket. The cable is also provided with strength members for rigidity.
U.S. Pat. No. 5,010,210 issued to Sidi et al. on Apr. 23, 1991 describes a non-plenum telecommunications cable wherein the insulation surrounding each of the conductors is formed from a flame retardant polyolefin base compound.
U.S. Pat. No. 5,162,609 issued to Adriaenssens et al. on Nov. 10, 1992 relates to a fire-resistant non-plenum cable for high frequency signals. Each metallic member has an insulation system. The insulation system includes an inner layer of a polyolefin and an outer layer of flame retardant polyolefin plastic.
U.S. Pat. No. 5,253,317 issued to Allen et al. on Oct. 12, 1993 describes a non-halogenated plenum cable including twisted pairs of insulated metallic conductors. The insulating material is a non-halogenated sulfone polymer composition. The insulating material is the same for all of the metallic conductors.
It can thus be understood that much work has been dedicated to providing not only communication cables that meet certain safety requirements but meet electrical requirements as well. Nevertheless, the most common communication cable that is in widest use today includes a plurality of twisted pairs of electrical conductors each having an insulation of FEP, which is a very high temperature material and possesses those electrical characteristics, such as, low dielectric constant and dissipation factor, necessary to provide high quality communications cable performance. However, FEP is quite expensive and is frequently in short supply.
Consequently, the provision of a communication cable for use in plenums but has a reduced cost and reduced use of FEP is highly desired.